Photoelectron spectroscopy, especially XPS, can be used to study some aspects of the surface chemistry of cuprates. Examples of this are the changes seen in the photoelectron spectra of Y123 as a freshly prepared surface deteriorates with time at 300 K in ultrahigh vacuum. In this, the evidence that the changes are due to loss of oxygen is not direct. A better example, to be discussed below, is the study of a metal overlayer on a cuprate. The overlayer may just sit there, making it a candidate for an ohmic contact, or it may disrupt the surface, replacing some of the atoms in the cuprate, which then appear in the surface layer. The appearance or disappearance of core-level peaks can be used to track the depths of constituents, and binding-energy shifts may give clues about oxidation states and location of atoms, i.e., surface or bulk.

Most of the studies of interface reactions have been carried out on the same type of surfaces used in the study of the cuprates themselves, cleaved single crystals or scraped or fractured polycrystalline samples. These cannot be considered technologically important surfaces unless processing in the future is carried out under ultrahigh vacuum conditions. Some of the studies reported below may need to be repeated on air-exposed surfaces, or surfaces handled in inert atmospheres or poorer vacua.